A known track comprises geometrical discontinuities formed on the periphery of the shaft (holes, fluting, studs).
However, it is expensive to machine such discontinuities.
It is also known (DE-A-3 109 586) to use a track fixed to the shaft and constituted by a strip of electrical insulation material including areas of non-ferromagnetic material having good electrical conductivity, said areas being regularly distributed around the shaft.
In the prior system, the areas of non-ferromagnetic material are disposed on the electrical insulating material strip facing away from the shaft.